In many mature Oil fields, most existing low-productivity wells, also called xe2x80x9cstripper wellsxe2x80x9d, become un-economic when oil prices drop below $14/B, thus causing their premature abandonment and the loss of their remaining Petroleum reserves. To prevent this loss of a precious Natural Resource, it is necessary to boost the wells productivity at low Capital Cost, without any significant increase of the wells Operating Cost.
A proven method of reaching the objective of an increased well productivity is to convert single wells into multi-lateral wells. These drain a larger area of the reservoir, either because the added branch well is drilled into a different layer or because it is highly deviated to reach an un-depleted region of the original productive layer. Various types of downhole sealed connectors have been described and claimed in U.S. Pat. No. 5,462,120, but the present Invention is especially applicable to existing wells equipped with a casing of outside diameter ranging from 7⅝xe2x80x3 to 6.5xe2x80x3 and cemented or not at the lateral kick-off point. The pre-fabricated Assembly is designed so as to minimize the cost of its installation in the existing well, by reducing the required rig time, while providing both a reliable sealed connection of the casing with the branch well liner and full access to the bottom of the casing, below the kick-off point. These two main features are required whenever the existing and branch wells are not at comparable pressures or temperatures, because of reservoir or fluid characteristics, or when the two wells must be operated independently of each other, for instance to convey different fluids, as in the configurations described and claimed in U.S. Pat. No. 5,085,279. These features are not achievable for existing wells of those sizes, using any presently available connecting equipment.
Furthermore, the use of the Assembly, in conjunction with a Combined Apparatus for jet-drilling, and for the liner completion of the branch well through the sealed connection, provides additional cost saving benefits, for which conventional drilling tools of the required small size are not well suited, especially in relatively soft formations.
The first step required for making a branch lateral connection to an existing cased well is that a window be cut-out in the casing to provide access to drill strings and completion tubulars required for the branch well. Performing this operation with a milling bit at the end of a drill string is a time-consuming task. It also results in an irregular window""s edge providing a poor fit with the upper end of the branch well liner hung and sealed in a short connecting tube (called a liner stub).
The generally poor fit obtained between the liner stub and a conventionally milled-out casing window makes the sealed junction of the existing well with the new branch lateral entirely dependent upon the bonds between the steel of the two poorly fitting tubulars and the cement filling the gap between them.
The long-term integrity of such a cement to steel seal is unreliable when the well tubulars are subjected to cyclic stresses resulting from pressure or temperature variations at the junction of casing and liner stub, during operation of the dual well.
In addition to the high window-cutting cost, the conventional use of a succession of many different downhole tools requires many trips of the work string, which increase the total rig time and Capital cost of the work-over beyond the limit of affordability for marginal wells.
The present Invention addresses these problems by the design of a multi-function apparatus to be used in existing cased wells, called a xe2x80x9cLiner Stubxe2x80x9d Assembly, of outside diameter not exceeding the well casing drift diameter, such that said Assembly, used in the First, Second, Third and Fourth Embodiments of the Invention is designed to be:
1) factory pre-fabricated at low cost, from inexpensive drillable materials (except for the high-strength steel stub), including a housing equipped with an outer retrievable hanger-packer, and presenting an inner cavity containing said liner stub,
2) run-in, with the liner stub in a locked position, at the end of a 5xe2x80x3 or 4.5xe2x80x3 OD work string, oriented and set in the casing, preferably opposite a soft formation, all in a single trip,
In addition, said multi-function Assembly allows:
3) the insertion of the stub in a casing window neatly cut-out, in a very short time, using cordon-type linear explosive shaped charges, all equipped with appropriate cutting liners, disposed in a template also included in the said Assembly, but armed at the well site,
4) the remains of the casing wall left in the window and other large debris to be removed by wireline fishing tools, through the work string and the Assembly housing,
5) a side pocket hole of approximate dimensions sufficient to contain the liner stub to be drilled, prior to the liner stub""s full extension from the Assembly""s housing cavity through the casing window into said pocket hole, in which a cement slurry is displaced outside the extended liner stub, by conventional means,
6) the liner stub, when un-locked and fully extended into said pocket hole, to be at a prescribed small angle (typically less than one degree) from the axis of said Assembly, by using an associated stub-guiding system, also included in said Assembly,
7) the cordons"" sequential detonation, controlled by a surface-triggered firing system, included in said Assembly, to shatter the sand face within the cut-out window, followed by small-size debris removal to the surface by reverse circulation of the casing fluid, using flow channels included outside the housing of said Assembly, which may be used during the period of extension of the liner stub in said side pocket hole, and thereafter, during drilling and completion of the branch well,
8) a soft metal stop-collar, affixed at the annealed upper end of the liner stub during pre-fabrication of said Assembly, to reliably stop the liner stub""s extension and to maintain said stub in close contact with the inner surface of the casing, along the window""s edge, for re-inforcement of the casing and liner stub at their junction,
9) after full-extension of the stub, and displacement of a slug of cement slurry behind the stub in the hole, the guiding system of the liner stub and charge enclosure debris to be quickly retrieved, by wireline, directly through the work string and Assembly housing, or by drilling-out, using a smaller-diameter drill string, inserted in the work string,
10) secondary explosive charges, affixed to said liner upper end and protected by a drillable pressure-resistant annular enclosure to be independantly detonated by a second surface-triggered firing system, also included in said Assembly, as means of bonding the end of the liner stub and its metal collar to the casing, all along the window""s edge to form an explosively-welded, reliably leak-proof, metal seal between the casing and liner stub metals, capable of withstanding considerable stress,
11) All debris from the explosions and some of the stub guiding systems are removed, but most of the Assembly housing in the First and Second Embodiments remains, still supported by the hanger-packer in the casing. It is now used as a guide for the insertion of cleaning, cementation and completion tools into the explosively-welded stub. Conversely, in the Third Embodiment, a cement slurry is squeezed around the windowed part of the casing, after all debris from the cover plate and from the cut casing have been removed through the bent liner stub. The welded and cemented curved liner stub is now ready to serve as a tool guide for drilling, cementation and completion of the branch well and as a sealed anchor for its liner.
After the cement slurry displaced behind the casing and the welded liner stub has set, the cement plug at the bottom of the stub is drilled through, so as to begin drilling and completion of the branch well. This is advantageously done by means of a jet-drilling and liner positioning Combined Apparatus, which still includes a large large portion of the Assembly housing, its support in the casing and the large-diameter work string, required to run it, and the liner stub itself, after it has been explosively-welded to the casing and cemented in place.
This Apparatus is disclosed as the Fourth Embodiment of the Invention. It also includes a mud circulation system and a buoyant spoolable tubular umbilical, co-axial with a segment of coiled liner inserted, through the work string, via the Assembly housing and the installed liner stub, into the branch borehole, while it is being drilled, using a jet-driling process, derived in part from U.S. Pat. No. 5,402,855.
In a First Embodiment of the Invention, the Liner Stub Assembly, equipped with its stiffening internals, and its guiding system are preferably fabricated by the method disclosed in the Co-pending Pat. No. 6,065,209 (third embodiment).
In the Second Embodiment of the Invention, only the upper end of the Liner Stub, including its stiffening tie-rods, is fabricated by the method disclosed in the same Co-pending Patent.
In the Third Embodiment of the Invention, a Pre-curved Liner Stub Assembly and its associated by-pass tubing are used to reach even greater cost-saving objectives, but with a large reduction of the access to the original well bottom. This Pre-curved Liner Stub serves the same purpose as the straight Liner Stubs in the first two Embodiments, namely to provide an anchor and a sealed connection between the casing and the branch well liner.
Like the stubs of the First and Second Embodiments, the Pre-curved Liner Stub Assembly, including its stiffening internals, collar and cover plate are all fabricated by the method disclosed in said Co-pending Patent (see 4th embodiment of U.S. Pat. No. 6,065,209).
As in the first two Embodiments, the Pre-curved Liner Stub is explosively-welded to the casing, along the edge of the casing window, also cut with explosives, but their junction is now at the lower end, rather than at its upper end.
The Pre-curved Liner Stub, however, remains stationary within the casing, instead of being thrust into a side-pocket hole. This greatly simplifies its installation is the casing, but it also reduces access below the casing window. The only access to the casing space below the branch well is through a small-diameter by-pass tubing. Consequently, the Third Embodiment is applicable only to vertical cased wells of relatively low productivity.
Whereas the First Three Embodiments deal only with the Assembly used for constructing a branch connector, sealed to the casing of an existing well, the Fourth Embodiment deals with a combined Apparatus, including only a portion of the Assembly, the stub and the same work string. This Apparatus is used for drilling the branch borehole to its targeted depth, via the cemented connector, and for completion of the branch well with a coiled tubing liner.
This Combined Apparatus constitutes the Fourth Embodiment of the Invention.
It is used as tool guide, support and means of fluid circulation for the following three additional tasks of well construction:
12) drilling of the branch borehole, of diameter at least equal to that of the stub, preferably by means of a high-pressure jet, located at the end of a small diameter buoyant spoolable tubing, inserted in a segment of un-coiled metal liner terminated at its upper end by a tubing hanger and a packer, of diameter suitable for being set inside the cemented stub. The lower end of the liner is guided and supported in the highly-deviated hole, behind the drilling jet, by the buoyant lower end of the spoolable tubing. During the jet-drilling process, the respective penetrations of the liner segment and of the spoolable tubing are controlled hydraulically and mechanically from the surface,
13) after retrieval of the jet-drilling tools, the liner segment, suspended from the surface by a retrievable cable, is hung in the liner stub, gravel-packed, cemented and packed in the liner stub, ready for perforation by known means.
14) the suspension means of the liner, the work string, the remaining part of the Assembly and its retrievable support in the casing, are then removed, thus re-opening the casing above and below the window.
The dual well is then ready for installation of its tubings completion, by conventional means.
The use of said pre-fabricated stub Assembly, installed in a single trip of the work string, also provides cost-saving advantages for conventional operations included in the well work-over, subsequent to the explosive welding of the liner stub:
the same small-diameter drill string is used, in conjunction with the Assembly housing, to drill out excess cement in the stub and to begin drilling the deviated branch borehole via the welded stub. This may be done using either the rotary drilling method, or a downhole mud motor, or, preferably, the coiled tubing jet-drilling technology of U.S. Pat. No. 5,402,855, as part of the Combined Apparatus described above, in which the coiled tubing string is a, low-weight, spoolable, umbilical tubing.
The advantages presented by such a Combined Apparatus are:
the Assembly housing, in one or, preferably, two pieces, is included in said Combined Apparatus. It contributes to safely guiding small-diameter drilling tools and the liner string into the branch borehole, as well as conveying drilling or completion fluids, through the bonded casing-liner stub connection;
with the Assembly housing, reverse mud circulation from the annulus between casing and work string to the annulus between work string and umbilical tubing may be combined with a direct circulation from the umbilical tubing to the annulus between work string and umbilical tubing, resulting in improved cleaning of borehole, increased rate of penetration and easier insertion of the liner;
after reaching the targeted depth of the branch hole, the umbilical tubing is pulled-out, leaving in the Assembly only the liner string, made of a single 3.5xe2x80x3 OD coiled liner segment, preferably slotted in its lower part and hung in the welded and cemented liner stub;
Gravel packing of the annulus in the reservoir portion of the borehole, if required, and cementation of the liner in the upper part of the borehole, may proceed, through the Assembly and the work string; the liner packer is set in the liner stub;
the remainder of the Assembly housing may then be retrieved or drilled-out to restore access to the bottom part of the original casing.
The tubings completion of the dual well can then proceed, by conventional means.
Typically, a slick 2 xe2x85x9cxe2x80x3 OD threaded tubing or, preferably, a 2.25xe2x80x3 OD coiled tubing may be installed in the 4xe2x80x3 ID liner of the branch well. A parallel 2 xe2x85x9cxe2x80x3 OD tubing may be used in the original well, if the casing is 7xe2x80x3 OD or greater. A downhole pump and auxiliary flow control devices may also be included in the tubing completion of the dual well.
It is clear that the pre-fabricated liner stub Assembly and the Combined Apparatus, including a jet-drilling nozzle fed by a spoolable umbilical tubing, both contribute to reducing the number of trips and, correspondingly, the rig labor required for the complete work-over conversion of the existing well into a dual well, thus reducing its total Capital Cost.
The facts that access of logging and cleaning tools to the bottom of the casing is preserved and that totally independent operation of the two wells is possible, while sharing some of the original production equipment (casing, downhole pump, pumping unit, oil/water separator, gas handling piping, oil storage and water disposal system) at a single well site, all contribute to a reduction of the Operating and Maintenance Cost of the dual well, on a per-barrel basis, as compared with that of several, geographically-separated, conventional single wells, capable of a comparable cumulative production.
Because of these large savings, the preferred mode of a Branch Well Additionn to an existing cased well is to combine the use of anyone of the Assemblies disclosed in first three Embodiments, with the Combined Apparatus disclosed in the Fourth Embodiment.